Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One particular type of fluorescent lamp is a Cold Cathode Fluorescent Lamp (CCFL). CCFLs may be used for back or edge lighting of Liquid Crystal Displays (LCDs) which are typically used in laptop computers, web browsers, automotive and industrial instrumentation, and entertainment systems.
CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphoric material coated in the inner wall of the tube, resulting in visible light.
A power conversion circuit may be used for driving the CCFL. The power conversion circuit accepts a Direct Current (DC) supply voltage and provides a substantially sinusoidal output voltage to the CCFL. The brightness of the CCFL is controlled by controlling the current, i.e., the lamp current, through the CCFL. The lamp current may be amplitude modulated or time modulated for controlling the brightness of the CCFL. The lamp current may be time modulated by selectively turning off the sinusoidal output voltage provided to the CCFL for varying time durations. For example, the sinusoidal output voltage alternates between being on for TON seconds and being off for TOFF seconds. The period, i.e., the summation of TON and TOFF, may be generally fixed in constant frequency operation to reduce Electro-Magnetic-Field (EMF) interference with other devices. The on-time duty cycle (referring to the proportion of time that the sinusoidal output voltage is on as indicated in the equation TON/(TON+TOFF)) determines the brightness of the CCFL. Maximum brightness results when the sinusoidal output voltage is on all the time with a 100% duty cycle, i.e., TOFF=0. Minimum brightness results when the duty cycle is small, i.e., TOFF>>TON.
When the supply voltage (DC supply voltage) referred to above changes from a low voltage, e.g., 6 volts, to a high voltage, e.g., 22 volts, there may result a large transient current inputted to the CCFL. This large transient current may result in shortening the lifetime of the CCFL. For example, in certain applications such as a laptop computer, a user may use either a battery or an adapter, e.g., Alternating Current (AC) adapter, which may supply a different amount of voltage. For example, the battery may supply 6 volts; whereas, the adapter may supply 22 volts. If a user switches from using a low supply voltage, e.g., battery, to a high supply voltage, e.g., adapter, then a corresponding increased sinusoidal output voltage may be produced. This increased sinusoidal output voltage may produce a large transient current which is inputted to the CCFL thereby shortening the lifetime of the CCFL.
If, however, the CCFL could be protected from such a large transient current, then the lifetime of the CCFL may be prolonged.
Therefore, there is a need in the art to protect a cold cathode fluorescent lamp from a large transient current when the voltage supply transitions from a low voltage, e.g., 6 volts, to a high voltage, e.g., 22 volts.